Sacrificial compositions and methods of fabricating a structure using sacrificial compositions

ABSTRACT

Compositions, methods of use thereof, and methods of decomposition thereof, are provided. One exemplary composition, among others, includes a polymer and a catalytic amount of a negative tone photoinitiator.

CLAIM OF PRIORITY

This application claims priority to and is a divisional of co-pendingU.S. patent application No. 11/398,183, filed Apr. 5, 2006, which is adivisional of co-pending U.S. utility patent application entitled“Sacrificial Compositions, Methods of Use Thereof, and Methods ofDecomposition Thereof,” having application Ser. No. 10/699,330 and filedon Oct. 31, 2003, which claims priority to U.S. provisional applicationentitled “Novel Selective-Temperature Sacrificial Polymeric Materials”having Application No. 60/423,013, filed on Nov. 1, 2002, all of whichare entirely incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. government may have a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of MDA awardedby the National Science Foundation (Grant #DMI-9980804) of the U.S.Government.

TECHNICAL FIELD

The present invention is generally related to sacrificial compositions,and, more particularly, is related to sacrificial polymers and theirdecomposition catalysts, methods of use thereof, and methods ofdecomposition of the sacrificial polymers.

BACKGROUND

A wide spectrum of microelectronic and microelectromechanical systems(MEMS) applications has increased the need for lower-temperature,thermally decomposable sacrificial materials. This includes fabricationof air-gaps in electrical interconnects, MEMS, microfluidic devices, andmicro-reactors.

The formation of air-gaps is important in electrical interconnectsbecause it lowers the effective dielectric constant of the matrix. Thefabrication of buried air channels is useful for the creation of vias inmulti-level wiring boards, micro-display boards with high resolution,and ink-jet printer heads. In MEMS technology, the fabrication ofmicro-air cavities may alleviate the stress associated with thermalexpansion of materials and also can act as a temperature-activatedrelease material.

Microfluidic devices and microreactors, fabricated with air-gaptechnology can be used for miniature-scale chemical syntheses, medicaldiagnostics, and micro-chemical analysis and sensors. In such devices,liquids and gases are manipulated in microchannels with cross-sectionaldimensions on the order of tens to hundreds of micrometers. Processingin such microchannel devices offers a number of advantages including lowreagent and analyte consumption, highly compact and portable systems,fast processing times, and the potential for disposable systems.

In spite of all of their promise, however, microfluidic devices arecurrently being used in a limited number of applications and are ingeneral still rather simple devices in terms of their operationalcomplexity and capabilities. For example, in terms of making trulyportable microanalytical systems, one of the current difficultiesinvolves the simple integration of electronic (e.g., sensing methods)and fluidic elements into the same device. One of the most importantissues, controlling this ability to integrate functions into the samedevice, and thus controlling the level of functionality of amicrofluidic device, is the method used to fabricate the structure.

The applications for a microfluidic device require the formation ofburied microchannels in several different materials at a variety oftemperatures. Polycarbonates have been used as a sacrificial material infabricating nanofluidic devices by electron beam lithography. C. K.Harnett, et al., J Vac. Sci. Technol. B., vol. 19(6), p. 2842, 2001.Air-gaps have been also fabricated using the hot-filament chemical vapordeposition of polyoxymethylene as a sacrificial layer. L. S. Lee, etal., Electrochem. and Solid State Lett., vol. 4, p. G81, 2001. Further,highly structured, dendritic material, specifically hyperbranchedpolymers, have been used as a dry-release sacrificial material in thefabrication of a cantilever beam. H-J. Suh, et al., J. Microelectromech.Syst., Vol. 9(2), pp. 198-205, 2000. Previous work has also fabricatedair-gaps using non-photosensitive sacrificial polymers that decompose inthe range 250-425° C. P. A. Kohl, et al., Electrochemical and SolidState Lett., vol.1, p.49, 1998; D. Bhusari, et al., J Micromech.Microeng., vol.10(3), p. 400, 2001.

FIGS. 1A-1H are cross-sectional views that illustrate a previouslyproposed method 100 for forming a buried air cavity using anon-photosensitive sacrificial material. FIG. 1A illustrates a substrate10, prior to having a non-photosensitive sacrificial material 12disposed thereon by, for example, spin-coating as in FIG. 1B. FIG. 1Cillustrates a hard mask 14 disposed on the sacrificial material 12. FIG.1D illustrates a photolithographed and etched mask portion 16 disposedon the hard mask 14, while FIG. 1E illustrates the removal of the maskportion 16 and portions of the sacrificial polymer material 12 exposedto the plasma etching. The hard mask 14 is then removed, as shown inFIG. 1F.

FIG. 1G illustrates the formation of the overcoat layer 18 onto thesacrificial polymer 12 and the substrate 10. FIG. 1H illustrates thedecomposition of the sacrificial polymer 12 to form air-regions 20. Thesacrificial polymer 12 has conventionally been decomposed by heating thesacrificial polymer 12 to a temperature sufficient to decompose thepolymer (e.g., about 300-425° C.). Prior sacrificial polymers haverequired high decomposition temperatures (e.g., about 300-425° C.),which limits the types of overcoat materials and substrates that may beused, as the overcoat and substrate materials must be able to withstandthe high temperatures needed to decompose the sacrificial polymers. Eventhe use of a polycarbonate polymer as the sacrificial polymer lowers thedecomposition temperature to about 250-280° C.

Thus, a heretofore unaddressed need exists in the industry to addressthe aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

Briefly described, embodiments of this disclosure, among others, includepolymer compositions and methods of use thereof, particularly, methodsof fabricating a structure using sacrificial compositions. An exemplarycomposition, among others, includes a polymer and a catalytic amount ofa negative tone photoinitiator.

Methods of fabricating a structure are also provided. One exemplarymethod, among others, includes the steps of: disposing a compositiononto a surface, the composition including a sacrificial polymer and aphotoacid generator; exposing at least a portion of the composition ofthe composition to energy; and removing a portion of the composition toform an air-gap in the composition, the removed portion corresponding tothe portions exposed to the energy. Another exemplary method offabricating a structure includes following steps: disposing acomposition onto a surface, the composition including a sacrificialpolymer and a catalytic amount of a photoacid generator; exposing aportion of the composition to energy; and removing the portion of thecomposition exposed to energy to form an air-gap in the composition viaheating the composition to about 100 to 180° C.

Other compositions, methods, features, and advantages will be, orbecome, apparent to one with skill in the art upon examination of thefollowing drawings and detailed description. It is intended that allsuch additional compositions, methods, features, and advantages beincluded within this description, be within the scope of the presentinvention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of this disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of this disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIGS. 1A through 1H are cross-sectional views that illustrate apreviously proposed method for forming buried air-gaps formation using anon-photosensitive sacrificial material.

FIGS. 2A through 2F are cross-sectional views that illustrate arepresentative method for forming buried air-gaps using a polymer thatundergoes acid-catalyzed decomposition.

FIGS. 3A through 3F are cross-sectional views that illustrate analternative representative method for forming buried air-gaps using apolymer that undergoes acid-catalyzed decomposition.

FIGS. 4A through 4F illustrate cross-sectional views that illustrate analternative representative method for forming buried air cavities usinga polymer that undergoes negative-tone stabilization to make thesacrificial polymer more difficult to decompose.

FIG. 5 illustrates chemical structures of exemplary sacrificial polymersused in the methods of FIGS. 2-4.

FIG. 6 illustrates chemical structures of exemplary photoacid generatorsused in the methods of FIGS. 2-3.

FIG. 7 illustrates chemical structures of further exemplary photoacidgenerators used in the methods of FIGS. 2-3.

FIG. 8 is a thermogravimetric plot of various disclosed sacrificialcompositions.

FIG. 9 is a scanning electron microscope (SEM) micrograph of thelithographic images of an air-gap formed from a representative disclosedsacrificial composition.

FIG. 10 is a SEM micrograph of an air-gap formed from an alternativesacrificial composition.

FIG. 11 is a SEM micrograph of an air-gap formed from an alternativesacrificial composition.

FIG. 12 is a graph of infrared (IR) spectra for a representativedisclosed sacrificial composition during the various stages of arepresentative method of forming air channels.

FIG. 13 illustrates mass spectrometry (MS) scans of a representativedisclosed sacrificial composition after the composition has been exposedto ultraviolet (UV) radiation.

FIG. 14 illustrates MS scans of a representative disclosed sacrificialcomposition without exposure to UV radiation.

FIG. 15 illustrates selected MS scans for a representative sacrificialpolymer, and its related fragmentation products.

FIG. 16 depicts the chemical structures of other representative volatilecompounds that evolve from the degradation of a representativesacrificial polymer formulation.

FIG. 17 depicts the chemical structures of a mechanism for theacid-catalyzed decomposition of a representative sacrificial polymer.

FIG. 18 depicts the chemical structures of a mechanism for the aldolcondensation products evolved during the acid-catalyzed degradationreaction depicted in FIG. 17.

DETAILED DESCRIPTION

In general, polymers, methods of use thereof, structures formedtherefrom, and methods of decomposition thereof, are disclosed.Embodiments of the polymer can be used to form air-gaps in electricalinterconnects, microelectromechanical (MEMS), microfluidic devices, andmicro-reactors. In addition, methods are disclosed in which sacrificialcompositions are decomposed at reduced temperatures than traditionallyused.

Embodiments of the disclosed sacrificial composition include, but arenot limited to, a sacrificial polymer and one or more positive tone ornegative tone components. The positive tone component can include aphotoacid generator, for example.

In general, the photoacid generator can be used to make the sacrificialpolymer easier to remove (e.g., less stable towards a solvent). Forexample, half of a layer of a sacrificial composition (e.g., asacrificial polymer and a positive tone component) is exposed to energy,either in the form of thermal energy (e.g., increased temperature) oroptical energy (e.g., ultraviolet (UV) light, near-ultraviolet light,and/or visible light), while the other half is not exposed.Subsequently, the entire layer is exposed to a solvent or heat and thesolvent or heat dissolves the layer exposed to the energy.

Although not intending to be bound by theory, upon exposure to opticalenergy, a positive tone photoacid generator generates an acid. Then,upon exposure to a base or an increased temperature, the dissolution ofthe sacrificial polymer is increased relative to sacrificial compositionnot exposed to the optical or thermal energy. As a result, a mask, forexample, can be used to fabricate three-dimensional structures from thesacrificial composition by removing the exposed sacrificial polymer.

In general, negative tone compositions can be used making thesacrificial polymer more difficult to remove (e.g., more stable towardsa solvent or heat that normally would dissolve the sacrificial polymer).For example, half of a layer of a sacrificial composition (including asacrificial polymer and a negative tone photoinitiator) is exposed tooptical energy, while the other half is not exposed. Subsequently, theentire layer is exposed to a solvent or heat and the solvent or heatdissolves the layer not exposed to the optical energy.

More specifically, the area exposed includes a cross-linkedphotodefinable polymer, while portions not exposed include anuncross-linked photodefinable polymer. The uncross-linked photodefinablepolymer can be removed with the solvent leaving the cross-linkedphotodefinable polymer behind (e.g., a photodefinable three-dimensionalstructure).

Although not intending to be bound by theory, upon exposure to opticalenergy, one type, among others, of the negative tone photoinitiator cangenerate free radicals that initiate cross-linking reactions between thesacrificial polymers to form a cross-linked photodefinable polymer.Therefore, a mask, for example, can be used to fabricate photodefinablethree-dimensional structures from the photodefinable polymer by removingthe uncross-linked photodefinable polymer.

In general, the sacrificial composition can be used in areas such as,but not limited to, microelectronics (e.g., microprocessor chips,communication chips, and optoeletronic chips), microfluidics, sensors,analytical devices (e.g., microchromatography), as a sacrificialmaterial to create three-dimensional structures that can be subsequentlyhave formed therein air-regions (also referred to herein interchangeablyas “air-gaps,” “air cavities,” and/or “air channels”) by thermallydecomposing the sacrificial polymer. In addition, the sacrificialpolymer can be used as an insulator, for example.

For embodiments using the sacrificial composition to create air-regionshaving three-dimensional structures, the decomposition of thesacrificial composition should produce gas molecules small enough topermeate one or more of the materials surrounding the sacrificialcomposition (e g., an overcoat layer). In addition, the sacrificialcomposition preferably decomposes slowly, so as not to create unduepressure build-up while forming the air-region within the surroundingmaterials. Furthermore, the sacrificial composition desirably has adecomposition temperature less than the decomposition or degradationtemperature of the surrounding material. The sacrificial compositionalso desirably has a decomposition temperature above the deposition orcuring temperature of an overcoat material but less than the degradationtemperature of the components in the structure in which the sacrificialcomposition is being used.

The sacrificial polymer can include compounds such as, but not limitedto, polynorbornenes, polycarbonates, functionalized compounds of each, acopolymer of polynorbornene and polynorbornene carbonate, andcombinations thereof. The polynorbornene can include, but is not limitedto, an alkenyl-substituted norbornene (e.g., cyclo-acrylate norbornene).The polycarbonate can include, but is not limited to, polypropylenecarbonate (PPC), polyethylene carbonate (PEC), polycyclohexane carbonate(PCC), polycyclohexanepropylene carbonate (PCPC), and polynorbornenecarbonate (PNC), and combinations thereof. The chemical structures ofrepresentative sacrificial polymers included in the disclosedsacrificial composition are depicted in FIG. 5. Specific polycarbonatesthat may be used as the disclosed sacrificial polymer include, forexample,poly[(oxycarbonyloxy-1,1,4,4-tetramethylbutane)-alt-oxycarbonyloxy-5-norbornene-2-endo-3-endo-dimethane)];poly[(oxycarbonyloxy-1,4-dimethylbutane)-alt-(oxycarbonyloxy-5-norbornene-2-endo-3-endo-dimethane)];poly[(oxycarbonyloxy-1,1,4,4-tetramethylbutane)-alt-(oxycarbonyloxy-p-xylene)]; andpoly[(oxycarbonyloxy-1,4-dimethylbutane)-alt-(oxycarbonyloxy-p-xylene)]. In general, the molecular weight of thedisclosed sacrificial polymers is from about 10,000 to 200,000.

The sacrificial polymer can be from about 1% to 50% by weight of thesacrificial composition. In particular, the sacrificial polymer can befrom about 5% to 15% by weight of the sacrificial composition.

As mentioned above, the sacrificial composition can include either anegative tone component and/or a positive tone component. The negativetone component can include compounds that generate a reactant that wouldcause the crosslinking in the sacrificial polymer. The negative tonecomponent can include compounds, such as, but not limited to, aphotosensitive free radical generator. Alternative negative tonecomponents can be used, such as photoacid generators (e.g., inepoxide-functionalized systems).

A negative tone photosensitive free radical generator is a compoundwhich, when exposed to light breaks into two or more compounds, at leastone of which is a free radical. In particular, the negative tonephotoinitiator can include, but is not limited to,bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819, CibaSpecialty Chemicals Inc.);2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (Irgacure369, Ciba); 2,2-dimethoxy-1,2-diphenylethan-1-one (Irgacure 651, Ciba);2-methyl-1[4-(methylthio) -phenyl]-2-morpholinopropan-1-one (Irgacure907, Ciba); benzoin ethyl ether (BEE, Aldrich);2-methyl-4′-(methylthio)-2-morpholino-propiophenone;2,2′-dimethoxy-2-phenyl -acetophenone (Irgacure 1300, Ciba);2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone (BAC-E), andcombinations thereof.

The positive tone component can include, but is not limited to,photoacid generator(s). More specifically, the positive tone photoacidgenerator can include, but is not limited to, nucleophilic halogenides(e.g., diphenyliodonium salt, diphenylfluoronium salt) and complex metalhalide anions (e.g., triphenylsulphonium salts). In particular, thephotoacid generator can betetrakis(pentafluorophenyl)borate-4-methylphenyl[4-(1-methylethyl)phenyl]iodonium(DPI-TPFPB); tris(4-t-butylphenyl)sulfoniumtetrakis-(pentafluorophenyl)borate (TTBPS-TPFPB);tris(4-t-butylphenyl)sulfonium hexafluorophosphate (TTBPS-HFP);triphenylsulfonium triflate (TPS-Tf); bis(4-tert-butylphenyl)iodoniumtriflate (DTBPI-Tf); triazine (TAZ-101); triphenylsulfoniumhexafluoroantimonate (TPS-103); Rhodosil™ Photoinitiator 2074 (FABA);triphenylsulfonium bis(perfluoromethanesulfonyl) imide (TPS-N1);di-(p-t-butyl) phenyliodonium bis(perfluoromethanesulfonyl) imide(DTBPI-N1); triphenylsulfonium; tris(perfluoromethanesulfonyl)methide(TPS-C1); di-(p-t-butylphenyl)iodoniumtris(perfluoromethanesulfonyl)methide (DTBPI-C1); and combinationsthereof, the chemical structures of which are depicted in FIGS. 6 and 7.

The photoacid generator can be from about 0.5% to 5% by weight of thesacrificial composition. In particular, the photoacid generator can befrom about 1% to 3% by weight of the sacrificial composition.

The remaining percentage of the sacrificial composition not accountedfor in the photoacid generator and sacrificial polymer (e.g., from about50% to about 99%) can be made up with solvent, such as, but not limitedto, mesitylene, N-methyl-2-pyrrolidinone, propylene carbonate, anisole,cyclohexanone, propylene glycol monomethyl ether acetate, N-butylacetate, diglyme, ethyl 3-ethoxypropionate, and combinations thereof.

Exemplary sacrificial polymers include those chemical structuresdepicted in FIG. 5. When using sacrificial polymers to fabricate air-gapstructures, it is helpful to know the degradation temperatures of thesacrificial polymers. Table 1 below shows the half-decompositiontemperatures, T_(1/2d) (where T_(1/2d) is the temperature at 50% weightloss) of exemplar sacrificial polymers under various experimentalconditions, determined using dynamic thermogravimetric analysis (TGA).

TABLE 1 Half-decomposition temperatures, T_(1/2d) (where T_(1/2d) is thetemperature at 50% weight loss) of exemplar sacrificial polymers undervarious conditions. PIAD TIAD TD Polycar- T_(1/2d) T_(1/2d) T_(1/2d)Entry bonate Solvent PAG (° C.) (° C.) (° C.) 1. 12% Anisole DPI-TPFPB102 ± 2 180 ± 2 212 ± 2 PPC 2. 20% Anisole TTBPS-TPFPB 125 ± 2 214 ± 2212 ± 2 PPC 3. 10% Propylene DPI-TPFPB 114 ± 2 189 ± 2 182 ± 2 PECcarbonate 4. 10% Propylene TTBPS-HFP 117 ± 2 184 ± 2 182 ± 2 PECcarbonate 5. 12% Anisole DPI-TPFPB 177 ± 2 199 ± 2 270 ± 2 PCC 6. 35%Anisole TTBPS-TPFPB 191 ± 2 252 ± 2 270 ± 2 PCC 7. 20% Anisole DPI-TPFPB160 ± 2 194 ± 2 252 ± 2 PCPC 8. 20% Anisole TTBPS-TPFPB 186 ± 2 249 ± 2252 ± 2 PCPC 9. 12% Anisole DPI-TPFPB 207 ± 2 194 ± 2 305 ± 2 PNC 10.50% Anisole TTBPS-TPFPB 196 ± 2 291 ± 2 304 ± 2 PNC

Now having described the sacrificial composition in general, thefollowing describes exemplar embodiments for using the sacrificialcomposition to produce three-dimensional structures, where thethree-dimensional structures can be decomposed to form air-regions(e.g., a gas filled region substantially excluding a solid or liquidmaterial or a vacuum-region).

In general, disposing a layer of the sacrificial composition onto asubstrate and/or layer of material on the substrate can produce athree-dimensional structure. A mask is disclosed on or above thesacrificial composition or portions thereof that encodes the shape ofthe three-dimensional structure, as described below. After exposing thesacrificial composition through the mask to optical and/or thermalenergy and removing the unexposed sacrificial composition (negativetone) or the exposed sacrificial composition (positive tone), thethree-dimensional structure is formed.

The mask encodes a density profile that defines the three-dimensionalstructure. Upon exposure of the mask to optical and/or thermal energy, aknown amount of the energy is allowed to pass through portions of themask. The design of the mask is used to control the amount of energyallowed to pass through the mask. In particular, the mask can bedesigned to control the amount of energy allowed to pass through themask as a function of the position on the mask. Thus, the mask can bedesigned and used to produce the three-dimensional structure from thesacrificial composition by altering the amount of energy allowed to passthrough the mask as a function of the position on the mask. The mask canbe formed by methods known in the art (e.g., U.S. Pat. No. 4,622,114).

The three-dimensional structures (and the corresponding air-regions) canhave cross-sectional areas section such as, but not limited to,non-rectangular cross-sections, asymmetrical cross-sections, curvedcross sections, arcuate cross sections, tapered cross sections, crosssections corresponding to an ellipse or segment thereof, cross sectionscorresponding to a parabola or segment thereof, cross sectionscorresponding to a hyperbola or segment thereof, and combinationsthereof. For example, the three-dimensional structures can include, butare not limited to, non-rectangular structures, non-square structures,curved structures, tapered structures, structures corresponding to anellipse or segment thereof, structures corresponding to a parabola orsegment thereof, structures corresponding to a hyperbola or segmentthereof, and combinations thereof. In addition, the three-dimensionalstructures can have cross-sectional areas having a spatially-varyingheight. Although not illustrated, a non-rectangular, tapered, andasymmetrical air-region can be formed in conjunction with otherair-regions and/or air-channels to form microfluidic devices, sensors,and analytical devices, for example.

An exemplar embodiment of a device with air-regions 250 formed thereinin shown in cross-section in FIG. 2F. The device may include a substrate210, and an overcoat layer 240.

A substrate 210 on which the sacrificial composition is disposed can beused in systems such as, but not limited to, microprocessor chips,microfluidic devices, sensors, analytical devices, and combinationsthereof. Thus, the substrate 210 can be made of materials appropriatefor the particular desired system or device. Exemplar materials,however, include, but are not limited to, glasses, silicon, siliconcompounds, germanium, germanium compounds, gallium, gallium compounds,indium, indium compounds, or other semiconductor materials and/orcompounds. In addition, the substrate 210 can include non-semiconductorsubstrate materials, including any dielectric material, metals (e.g.,copper and aluminum), ceramics, or organic materials found in printedwiring boards, for example.

The overcoat layer 240 can be a modular polymer that includes thecharacteristic of being permeable or semi-permeable to the decompositiongases produced by the decomposition of a sacrificial polymer whileforming the air-region 250. In addition, the overcoat layer 240 haselastic properties so as to not rupture or collapse under fabricationand use conditions. Further, the overcoat layer 240 is stable in thetemperature range in which the sacrificial composition decomposes.

Examples of the overcoat layer 240 include compounds such as, but notlimited to, polyimides, polynorbornenes, epoxides, polyarylenes ethers,polyarylenes, inorganic glasses, and combinations thereof. Morespecifically the overcoat layer 240 includes compounds such as AmocoUltradel™ 7501, Promerus Avatrel™ Dielectric Polymer, DuPont 2611,DuPont 2734, DuPont 2771, DuPont 2555, silicon dioxide, silicon nitride,and aluminum oxide. The overcoat layer 240 can be deposited onto thesubstrate 210 using techniques such as, for example, spin coating,doctor-blading, sputtering, lamination, screen or stencil-printing,chemical vapor deposition (CVD), metalorganic chemical vapor deposition(MOCVD), and plasma-based deposition systems.

It should be noted that additional components could be disposed onand/or within the substrate 210, the overcoat layer 240, and/or theair-region 250. In addition, the additional components can be includedin any structure having air-regions as described herein. The additionalcomponents can include, but are not limited to, electronic elements(e.g., switches and sensors), mechanical elements (e.g., gears andmotors), electromechanical elements (e.g., movable beams and mirrors),optical elements (e.g., lens, gratings, and mirror), opto-electronicelements, fluidic elements (e.g., chromatograph and channels that cansupply a coolant), and combinations thereof.

FIGS. 2A-2F are cross-sectional views that illustrate a representativemethod 200 for forming an air-region using a sacrificial composition,the sacrificial composition including a sacrificial polymer and apositive tone component. FIG. 2A illustrates a substrate 210, prior tohaving a sacrificial material 220 disposed thereon by, for example,spin-coating as shown in FIG. 2B. FIG. 2C illustrates a mask 230disposed on or above the sacrificial material 220 and the sacrificialmaterial 220 being subjected to energy/radiation. While FIG. 2C depictsthe energy being applied in the form of UV radiation, other forms ofoptical energy or thermal energy can be applied.

FIG. 2D illustrates the removal of selected portions of the sacrificialcomposition 220 that was exposed to the UV radiation of FIG. 2C throughthe mask 230. The mask 230 encodes an optical density profile thatdefines the cross-section of the air-region 250. The removed portions ofthe sacrificial composition 220 are removed by heating the sacrificialcomposition. The temperature used to remove irradiated portions of thesacrificial material 220 is lower than the methods of the prior art, dueto the presence of a photoacid generator, described above, in thesacrificial composition 220. The temperature required to substantiallydecompose the irradiated sacrificial polymer 220 in the step shown inFIG. 2C is about 50° C. to 400° C. For example, the temperature may beabout 110° C.

FIG. 2E illustrates the formation of the overcoat layer 240 onto thesacrificial polymer 220 and exposed substrate 210. FIG. 2F illustratesthe decomposition of the sacrificial polymer 220 to form the air-regions250. The temperature required to substantially decompose the sacrificialpolymer in the step shown in FIG. 2F is about 100° C. to 250° C. Forexample, the temperature may be about 170° C.

FIGS. 3A-3F are cross-sectional views that illustrate an alternativemethod 300 for forming an air-region 250 using a sacrificialcomposition, the sacrificial composition including a sacrificial polymerand a positive tone component. FIG. 3A illustrates a substrate 210,prior to having a sacrificial material 220 disposed thereon by, forexample, spin-coating as shown in FIG. 3B. FIG. 3C illustrates a mask230 disposed on or above the sacrificial material 220 and thesacrificial material 220 being subjected to energy/radiation. While FIG.3C depicts the energy being applied in the form of UV radiation, otherforms of optical energy, or thermal energy, can be applied.

FIG. 3D illustrates the cross-linked sacrificial composition region 225after exposure through the mask 230 to optical energy. FIG. 3Eillustrates the formation of the overcoat layer 240 onto the sacrificialpolymer 220 and cross-linked composition regions 225. FIG. 3Fillustrates the decomposition of the un-crosslinked sacrificial polymer220 to form the air-regions 250. The removed portions of the sacrificialcomposition 220 are removed by heating the sacrificial composition. Thetemperature used to remove irradiated portions of the sacrificialmaterial 220 in FIG. 3F is lower than the methods of the prior art, dueto the presence of a photoacid generator, described above, in thesacrificial composition 220.

FIGS. 4A through 4F are cross-sectional views that illustrate arepresentative process 400 for fabricating the air-regions 250illustrated in FIG. 4F. It should be noted that for clarity, someportions of the fabrication process are not included in FIGS. 4A through4F. As such, the following fabrication process is not intended to be anexhaustive list that includes all steps required for fabricating theair-regions 250. In addition, the fabrication process is flexiblebecause the process steps may be performed in a different order than theorder illustrated in FIGS. 4A through 4F, or some steps may be performedsimultaneously.

FIG. 4A illustrates the substrate 210. FIG. 4B illustrates the substrate210 having the sacrificial composition 260 (negative tone) disposedthereon. The sacrificial composition 260 can be deposited onto thesubstrate 210 using techniques such as, for example, spin coating,doctor-blading, sputtering, lamination, screen or stencil-printing, meltdispensing, chemical vapor deposition (CVD), metalorganic chemical vapordeposition (MOCVD), and plasma-based deposition systems.

FIG. 4C illustrates a mask 230 disposed on or above the sacrificialcomposition 260. The mask 230 encodes an optical density profile thatdefines the cross-section of the photodefinable air-region 250.

FIG. 4D illustrates the cross-linked sacrificial composition region 270after exposure through the mask 230 to optical energy, after the removalof the selected uncross-linked sacrificial composition material 270. Theuncross-linked sacrificial composition material 260 can be removed bydissolution in a liquid, such as a solvent and/or heat, for example, orby any other method that can remove or dissolve the polymer component ofthe sacrificial composition material 260.

FIG. 4E illustrates the formation of the overcoat layer 240 onto thecross-linked sacrificial composition region 270. The overcoat layer 240can be deposited onto the substrate 210 using techniques such as, forexample, spin coating, doctor-blading, sputtering, lamination, screen orstencil-printing, melt dispensing, chemical vapor deposition (CVD),metalorganic chemical vapor deposition (MOCVD), and plasma-baseddeposition systems.

FIG. 4F illustrates the decomposition of the cross-linked sacrificialcomposition region 270 to form the air-region 250. The cross-linkedsacrificial composition region 270 can be decomposed by heating thecross-linked sacrificial composition 270 to a temperature sufficient todecompose the polymer (e.g., about 250° C.).

Thermogravimetric Analysis of Representative Sacrificial CompositionFilm

FIG. 8 illustrates the dynamic thermogravimetric (TGA) results for anexemplar sacrificial composition of a sacrificial polymer, polypropylenecarbonate (PPC), and a photoacid generator (PAG). The composition wascomprised of approximately 12 wt % PPC and 5 wt % DPI-TPFPB of PPC(“PPC:PAG”). The representative samples were prepared by spin-coatingthe polymer in anisole onto a silicon wafer, soft baking on a hotplateat about 110° C. for 10 minutes to evaporate the solvent and thenexposing, where applicable, to 1 J/cm² of 240 nm UV irradiation. Thesamples were then removed from the silicon and analyzed by dynamic TGAramping from 30 to 450° C. at the rate of 1° C. under a nitrogenatmosphere.

FIG. 8 shows the TGA thermograms for (a) PPC:PAG after UV irradiation,(b) PPC:PAG without UV irradiation, (c) PPC without PAG, and (d) PPCwithout PAG but with UV irradiation. As shown in FIG. 8( a), the onsettemperature for the photo-acid induced decomposition was found to be 80°C., and the T_(1/2d) was 100° C. The decomposition was complete at 230°C. with less than 3 wt % residue in the TGA pan.

When the sacrificial polymer material was not UV irradiated (FIG. 8(b)), decomposition occurred via the thermolytically induced PAG acidgeneration once the temperature reached the decomposition temperature ofthe PAG. The decomposition occurred in a narrow temperature range fromthe onset at 168° C. At 230° C., the thermal-acid induced decompositionwas complete with less than 1.5 wt % residue.

The decomposition behavior of PPC without PAG is shown in FIG. 8( c),and the T_(1/2d) was found to be 210° C. At 230° C., 19 wt % of the massremained in the pan. The decomposition was complete at 287° C. with 3 wt% residue. At 350° C., the weight percentages of the residues were1.79%, 0.12% and 0.37% for (a), (b), and (c), respectively, under thesame experimental conditions.

The decomposition behavior of the sacrificial polymer (here, PPC) filmwithout PAG but exposed to UV irradiation (FIG. 8( d)) was found to besimilar to that of the non-UV irradiated sacrificial polymer film. Thisshows that no change occurred in the sacrificial polymer upon UVirradiation without PAG. Thus, the photo-acid induced decomposition ofthe polycarbonates significantly lowers the decomposition temperatures;however, it leaves more residue than the thermolytically inducedacid-catalyzed decomposition or thermolytic decomposition ofpolypropylene carbonate alone. The residue level can be brought to lessthan 1 wt % by lowering the percentage of PAG level in the formulationfor the acid-catalyzed degradation.

Photopatterning and Fabrication of Air-Gaps Using PhotosensitiveSacrificial Compositions

Positive-tone patterns obtained using PPC (12 wt %):DPI-TPFPB (5 wt % ofPPC) formulation in anisole. The solution was spin-coated onto a siliconand soft-baked on a hotplate at 110° C. for 10 min to achieve athickness of 1.58 μm. The film was UV irradiated through a clear-fieldquartz mask with a dose of 1 J/cm² (240 nm). The film was dry-developedby post-exposure baking at 110° C. on a hotplate for 3 min. At 110° C.,the UV generated acid induced the decomposition of the PPC. FIG. 9 showsscanning electron microscope (SEM) micrographs of lithographic imagesproduced using a dark-field mask. The process was repeated as aboveexcept a PPC (20 wt %):DPI-TPFPB (5 wt % of PPC) in anisole formulationwas used to achieve a 5.45 μm thick film. The circles and squares arethe areas in which the PPC has been decomposed and removed.

Examples of Implementations of Disclosed Methods

Air-gaps were fabricated via the process flow using method 200 depictedin FIG. 2 using photosensitive polycarbonates as the sacrificialpolymeric material. Two different encapsulating materials were used:Avatrel EPM™ and Avatrel 2000P™ (Promerus, LLC.). First, thephotosensitive PPC (12 wt %):DPI-TPFPB (5 wt % of PPC) formulation wasspin-coated onto a silicon wafer and soft-baked on a hotplate at 100° C.for 10 minutes to achieve a thickness of 5.45 μm. The photosensitivefilm was photopatterned using a clear-field mask having lines/spacepattern with 70-μm wide lines and 35-μm wide spaces. Post-exposurebaking at 110° C. for 1-10 min. decomposed the UV irradiated area. TheAVATREL EPM encapsulating material was spin-coated and soft-baked on ahotplate at 80° C. for 5 minutes. The unexposed area was decomposed in aLindberg horizontal tube furnace at 150° C. for 4 hours under nitrogen.FIG. 10 shows a cross-sectional SEM micrograph image of the 70-μm wideresulting air channel structure encapsulated by 3.9 μm AVATREL EPM™dieletric material. A similar fabrication process sequence was followed,using AVATREL 2000P™ as the encapsulating material. However, afterspin-coating AVATREL 2000P, the film was UV irradiated with 1 J/cm² at365 nm and post-exposure baked in an oven at 110° C. for 30 minutes.Upon baking, the AVATREL 2000P™ polymer was cross-linked. The resultingthickness of the overcoat was 9.3 μm. The unexposed sacrificial polymerwas then decomposed in a Lindberg horizontal tube furnace at 170° C. for1 hour under nitrogen.

FIG. 11 shows an SEM image of the resulting 70-μm wide buriedair-channel in AVATREL 2000P™ encapsulant. The air channels were cleanfrom visible debris, which shows the permeability of the volatilesthrough the encapsulating polymeric material, or “overcoat” material.

Air channels were also fabricated via the process flow using method 300described in FIG. 3 using a PPC:TTBPS-TPFPB formulation as thesacrificial composition. The PAG, TTBPS-TPFPB, was chosen because itthermally decomposes at a higher temperature than the DPI-TPFPB. Ahigher decomposition temperature PAG leaves intact the unexposed regionof the sacrificial material, while the exposed area is selectivelydecomposed. The decomposition temperature of TTBPS-TPFPB was found to be190° C. from the differential scanning calorimetry (DSC). Buriedair-channels in AVATREL EPM encapsulant were fabricated using theprocess sequence described as method 300 of FIG. 3. A PPC (20 wt%):TTBPS-TPFPB (5 wt % of PPC) formulation was spin-coated onto asilicon wafer and soft-baked on a hotplate at 110° C., resulting in athickness of 5.45 μm. The photosensitive PPC film was irradiated (1J/cm²; 240 nm) through a clear-field mask having with a line/spacepattern of 100-μm wide lines and 240-μm wide spaces. The AVATREL EPMencapsulant was then spin-coated and soft-baked at 80° C. for 5 minutesto remove any solvent from the encapsulating layer. The exposed area wasthen selectively decomposed by heating at 110° C. for 30 minutes in atube furnace under nitrogen to form air-gaps.

FT-IR Analysis of the Decomposition of Representative SacrificialComposition Film

The acid catalyzed decomposition of PPC (12 wt %):DPI-TPFPB PAG (5 wt %of PPC) was studied by FT-IR. The sacrificial composition was spun ontoa NaCl plate and soft-baked on a hotplate at 110° C. for 10 min. Thethickness of the film was measured to be 1.45 μm. The FourierTransform-Infrared (FT-IR) spectrum of the unexposed film was recordedas shown in FIG. 12( a). The film was then exposed to UV light (1 J/cm²;240 nm) and scanned again (FIG. 12( b)). The film was post-exposurebaked on a hotplate at 110° C. for 2 h (FIG. 12( c)). In FIG. 12( a),the absorptions at 2990 cm⁻¹, 1470 cm⁻¹ and 1250 cm⁻¹ were assigned toC—H stretches, C—H bending and C13 C stretching of the PPC respectively.A strong absorption band at 1750 cm⁻¹ corresponds to C═O stretch of thePPC. On examining the UV-exposed PPC film (FIG. 12( b)), no specifictransformation in the chemical structure has occurred, except that thephotosensitive acid generator was activated during irradiation. Theintensities of all peaks were nearly zero after the final bake at 110°C. (FIG. 12( c)). This is believed to be due to the decomposition of thesacrificial polymer PPC into volatile products.

Decomposition of Representative Sacrificial Composition Monitored byMass Spectrometry

Mass spectrometry (MS) using electron impact ionization was employed todetect the evolved species during the depolymerization andvolatilization process for an exemplar sacrificial composition. Threerepresentative samples were analyzed to determine the nature of thechemical species that were produced during degradation. In the firstcase, the PPC (12 wt %): DPI-TPFPB (5 wt % of PPC) formulation wasspin-coated onto a silicon wafer, soft-baked on a hotplate at 110° C.for 10 minutes, then irradiated with 1 J/cm² UV irradiation at 240 nm.In the second case, the sample was prepared as above, but was not UVirradiated. In the third case, to study the volatiles evolved from thesacrificial polymer (here, PPC) without a photoacid generator, a samplewas made from the solution that contained only PPC. This PPC film wasnot UV irradiated. The films were removed from the silicon and analyzedby gas chromatograph-mass spectrometer (GC-MS) to study the evolution ofvolatiles under different conditions. The samples were ramped at 5°C./min from 30 to 110° C. and held for 30 minutes, followed by a ramp at15° C./min to 300° C. and held for 20 min.

The mass spectrum data for the decomposition of PPC:DPI-TPFPBformulation with UV exposure shows the ion current for a mass-to-chargeratio (m/z) of 58 and 102 which correspond to acetone and propylenecarbonate, respectively. Scans of the UV irradiated photosensitivepolycarbonate sample are shown in FIG. 13( a), (b) & (c). These scans atdifferent intervals of time during the degradation process reveal theevolution of these two prominent species. FIG. 13( a) also shows otherspecies at m/z 43.1, 40.1 and 36.1 which corresponds to thefragmentation products of acetone. The m/z 58 peak may also correspondto the formation of CH₃CH₂CHO. FIG. 13( b) shows other high intensityvolatiles corresponding to the molecular weights 43, 57, and 87, whichare the fragmentation products of the propylene carbonate and may beassigned to ethylene oxide, propylene oxide, and ethylene carbonate,respectively. The mass spectrum at 63° C. is presented in FIG. 13( c).The high intensity peak at 168 m/z can be assigned to a decompositionfragment of the PAG. This matches the mass of pentafluorobenzene (HC₆F₅)which is the derivative fragment of the anion of the photoacid,tetrakis(pentafluorophenyl)borate [B(C₆F₅)₄]. This was confirmed bypyrolysing the photoacid (dissolved in anisole) which exhibited a strongmass peak at 168 m/z at 67° C. Tetrakis(pentafluorophenyl)borate anioncan decompose on heating to yield pentafluorobenzene andtri(pentaflurophenyl)borate. The reaction includes the volatiles ofparticular interest that evolved from the degradation of thepolycarbonate formulation.

The suspected reaction products are shown in FIG. 16. The biphenylderivative (m/z 210) and the pentafluoro-benzene (m/z 168) resulted fromthe decomposition of the PAG, DPI-TPFPB. The other fragments having m/zvalues 137, 116, and 98 are the degradation products of the PPC. Athigher temperatures, small amounts of higher molecular weight fragmentswere observed. These may have resulted from more complex fragmentationpatterns.

The mass spectrum of the PPC formulation without UV exposure is shown inFIG. 14 and shows similar fragmentation patterns with that of PPCformulation with UV irradiation. Specifically, m/z peaks at 58 and 102were observed, corresponding to acetone and propylene carbonate,respectively. The peak at m/z=58 may also correspond to CH₃CH₂CHO. Thisconfirms that the acid catalyzed PPC decomposition is the same forphotolytic or thermal activation of the PAG. The mass spectrum of thePPC without acid (and without UV irradiation) is shown in FIG. 15. Themass peak at m/z=102 at 145° C. can be assigned to propylene carbonate(as with the acid-catalyzed PPC). The other mass peaks at 87, 57, and 43can be assigned to the propylene carbonate fragments: ethylenecarbonate, propylene oxide, and ethylene oxide, respectively.

Mechanism for the Acid-Catalyzed Decomposition of RepresentativeSacrificial Polymer

In one study, the MS data shows that the degradation of an exemplarsacrificial polymer, e.g., polypropylene carbonate, is initiated by thein-situ acid generated from the PAG, either photolytically or by thermalheating. Without being bound by any theory, it is believed that thedegradation of PPC may take place in two stages: the scission reactionof the polymer chain followed by the unzipping reaction. Under pyrolysisat 180° C., it has been observed that polypropylene carbonate yieldedpropylene carbonate. Further, the propylene carbonate was fragmented asCO₂ and propylene oxide. The evolution of the two prominent species,acetone and propylene carbonate, from the degradation of polypropylenecarbonate formulations (with and without UV irradiation) led to thefollowing proposal of a dual-pathway degradation mechanism.

FIG. 17( a) describes the decomposition of the DPI-TPFPB PAG. Onexposure to 240 nm UV irradiation, the cation part of the PAG decomposesto produce a proton that pairs with the complex anion to form a proticacid. In this mechanistic scheme, —RH is a donor of protons either fromthe solvent or the polymer itself. The decomposition of iodonium saltscan be achieved by either UV exposure or by thermal breakdown of thecation/anion pair of the iodonium salt. Thus, the iodonium salt can actas a photo-acid generator as well as a latent thermal-acid generator. Ithas also been reported by that identical reaction products were obtainedwhen onium salts were decomposed either photolytically or thermally. J.V. Crivello, et al., J. Polym. Sci. Part A: Polym. Chem., vol. 21, p.97, 1983.

FIG. 17( b) describes the proposed mechanism for the acid-catalyzeddecomposition of polypropylene carbonate. During post-bake, the H⁺ fromthe generated acid (H⁺X⁻) protonates the carbonyl oxygen and furtherrearranges the polar transition state leading to the formation ofunstable tautomeric intermediates, [A] and [B]. From the mass spectrumwe observed a strong mass peaks at 58 m/z and 102 m/z which was assignedfor acetone and propylene carbonate, respectively. The acetone may beformed as the intermediate [A] (Path 1) rearranges and fragmenting asacetone and CO₂. The formation of propylene carbonate may be attributedto the intramolecular attack of the anion of the intermediate [B] (Path2a) leading to the formation of the cyclic propylene carbonate. Thisfurther breaks down thermally into propylene oxide and CO₂. The masspeaks at 43 m/z and 57 m/z confirm the formation of ethylene oxide andpropylene oxide, respectively.

It is also reasonable to propose an alternative route for the formationof propylene carbonate in this highly acidic environment. The H⁺ mayactivate the terminal double bond (Path 2b), leading to the formation ofa cyclic transition state which on further rearrangement and abstractionof a proton by the counter ion X⁻, may yield propylene carbonate.Further confirmation of the degradation via Path 1 is the formation ofaldol condensation products. The mass peaks at 116 m/z and 98 m/z may bedue to products that were produced due to the aldol condensation (FIG.18).

CONCLUSIONS

The fabrication of air-gaps via acid-catalyzed degradation of asacrificial polymer and a catalytic amount of a photoacid generator hasbeen demonstrated. Based on the FT-IR and mass spectral studies, adetailed mechanism for the polypropylene carbonate and the DPI-TPFPBsystem has been proposed. The decomposition behavior of severaldifferent sacrificial compositions has also been studied using TGA. Thelow decomposition temperatures provide a mechanism to tailor thedecomposition temperature through PAG concentration or sacrificialpolymer structure. By providing a processing method to selectivelydecompose sacrificial composition areas through photoexposure, thesematerials are a promising candidate for various microelectronic,microfluidic, and MEMS fabrication applications. In particular, theinsolubility of the disclosed sacrificial polymers in the solvents usedfor dielectric materials makes the combination especially valuable.

It should be emphasized that the above-described embodiments of thisdisclosure are merely possible examples of implementations, and are setforth for a clear understanding of the principles of this disclosure.Many variations and modifications may be made to the above-describedembodiments of this disclosure without departing substantially from thespirit and principles of this disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

1. A method for fabricating a structure, comprising: disposing acomposition onto a surface, wherein the composition includes asacrificial polymer and a catalytic amount of photoacid generator;exposing a portion of the composition to energy; and removing theportion of the composition exposed to energy to form an air-gap in thecomposition via heating the composition to about 100 to 180° C., wheresaid sacrificial polymer is chosen from a polycarbonate.
 2. The methodof claim 1, further comprising disposing a mask on or above thecomposition, the mask encoding a profile defining an air-gap to beformed in the composition.
 3. The method of claim 1, further comprisingremoving a portion of the composition not exposed to energy by heatingto a temperature of about 175 to 200° C.
 4. The method of claim 1,wherein exposing the composition to energy comprises decomposing anorganic cation of the photoacid generator, thus generating a strongBrØnsted acid.
 5. The method of claim 4, wherein exposing thecomposition to energy comprises thermolytically decomposing thesacrificial polymer with the BrØnsted acid.
 6. The method of claim 1,wherein said sacrificial polymer is selected from polypropylenecarbonate, polyethylene carbonate, polycyclohexane carbonate,polycyclohexanepropylene carbonate, and combinations thereof.
 7. Themethod of claim 1, wherein said sacrificial polymer is selected frompoly[(oxycarbonyloxy-1,1,4,4-tetramethylbutane)-alt-(oxycarbonyloxy-p-xylene)],poly[(oxycarbonyloxy-1,4-dimethylbutane)-alt-(oxycarbonyloxy-p-xylene)],and combinations thereof.